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Using data from NASA’s Cassini spacecraft, a team of planetary researchers led by Cornell University has produced a new global topographic map of Titan, the largest moon of Saturn.
Top: stereographic polar projections of Titan’s topography with the South Pole left and the North Pole right; Ontario Lacus and the each of the large northern seas are found in local depressions. Bottom: equicylindrical projection of Titan’s topography. All maps have been corrected for the geoid. Image credit: Corlies et al, doi: 10.1002/2017GL075518.
The new map, described in two papers in the December 16 issue of the journal Geophysical Review Letters, combines all of the Titan topography data from multiple sources.
The map reveals that Titan is a little bit flatter — more oblate — than was previously known.
It also reveals several new features on the hazy moon, including new mountains, none higher than 700 m.
It provides a global view of the highs and lows of Titan’s topography, which enabled the scientists to confirm that two locations in the equatorial region of the moon are in fact depressions that could be either ancient, dried seas or cryovolcanic flows.
“The main point of the work was to create a map for use by the scientific community,” said Paul Corlies, a doctoral student at Cornell University.
Using the new map’s topographical data, the scientists found that Titan’s three seas share a common equipotential surface, meaning they form a sea level, just as Earth’s oceans do.
Either because there’s flow through the subsurface between the seas or because the channels between them allow enough liquid to pass through, the oceans on Titan are all at the same elevation.
“We’re measuring the elevation of a liquid surface on another body 10 AU away from the Sun to an accuracy of roughly 40 cm,” said Dr. Alex Hayes, also from Cornell University.
“Because we have such amazing accuracy we were able to see that between these two seas the elevation varied smoothly about 11 m, relative to the center of mass of Titan, consistent with the expected change in the gravitational potential.”
“We are measuring Titan’s geoid. This is the shape that the surface would take under the influence of gravity and rotation alone, which is the same shape that dominates Earth’s oceans.”
The researchers also found that Titan’s lakes communicate with each other through the subsurface.
They measured the elevation of lakes filled with liquid as well as those that are now dry, and found that lakes exist hundreds of meters above sea level, and that within a watershed, the floors of the empty lakes are all at higher elevations than the filled lakes in their vicinity.
“We don’t see any empty lakes that are below the local filled lakes because, if they did go below that level, they would be filled themselves,” Dr. Hayes said.
“This suggests that there’s flow in the subsurface and that they are communicating with each other.”
“It’s also telling us that there is liquid hydrocarbon stored on the subsurface of Titan.”
The authors also found that the vast majority of Titan’s lakes sit in sharp-edged depressions that ‘look like you took a cookie cutter and cut out holes in Titan’s surface.
The lakes are surrounded by high ridges, hundreds of meters high in some places. They seem to be formed the way karst is on Earth, in places like the Florida Everglades, where underlying material dissolves and the surface collapses, forming holes in the ground.
The lakes on Titan, like Earth’s karst, are topographically closed, with no inflow or outflow channels. But Earth karst does not have sharp, raised rims.
The shape of the lakes indicates a process called uniform scarp retreat, where the borders of the lakes are expanding by a constant amount each time. The largest lake in the south, for example, looks like a series of smaller empty lakes that have coalesced or conglomerated into one big feature.
“But if these things do grow outward, does that mean you’re destroying and recreating the rims all the time and that the rims are moving outward with it? Understanding these things is in my opinion the lynchpin to understanding the evolution of the polar basins on Titan,” Dr. Hayes said.
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Paul Corlies et al. 2017. Titan’s Topography and Shape at the End of the Cassini Mission. Geophysical Review Letters 44 (23): 11,754-11,761; doi: 10.1002/2017GL075518
Alex Hayes et al. 2017. Topographic Constraints on the Evolution and Connectivity of Titan’s Lacustrine Basins. Geophysical Review Letters 44 (23): 11,745-11,753; doi: 10.1002/2017GL075468
